The present invention relates to a method of detecting minute flaws on the surface of a metal material, which comprises directing a collimated laser beam on the surface of the metal material, and measuring the change in quantity of light from the beam which is reflected or scattered by the metal surface.
When a laser beam is directed onto the surface of a metal material, a distribution of bright points which is called a speckle pattern occurs due to the interference with the reflected beam. The shape, size and type of flaws on the outer surface of the metal material to be inspected can be detected by measuring and analyzing the speckle pattern. Namely, when a finely collimated laser beam is spot-directed or scan-directed onto the surface of the metal material at a predetermined angle, the quantity of light from the reflected beam which forms a speckle pattern is reduced because it is scattered by the metal surface when flaws exist thereon.
According to a conventional method, the laser beam reflected by the surface of the metal to be inspected is directly put into a light detector through a lens or it is separated by a mirror and then put into a light detector, so as to measure the amount of decrease in intensity of the light due to the scattering of the reflected laser beam. (See, for example, "Iron and Steel Engineer", pages 67 to 70, June 1974.) However, since a speckle pattern includes information on the details of the surface condition of a sound portion of the surface to be inspected, the result of the measurment is seriously affected by changes in the condition of a sound portion thereof, or by the vibration of the surface to be inspected which occurs when the metal material to be inspected is moving. Thus, the flaw-detecting capability of the conventional method is extremely low. When the scattered component of the laser beam is detected by a conventional method, an image formed by the scattered beam due to a flaw may be very sharp and may be positioned in a limited space. Thus, flaws oriented in a certain direction may not be detected at all.